A continuing problem in high frequency communications networks, both in their installation and maintenance, is the necessity of assuring the integrity of the transmission line. This is particularly true in cable television systems where RF leaks in the CATV coaxial cable distribution networks can severely effect the quality of the received signal. Such leaks may be due to small cracks in the cable, loose fittings or other non-RF tight conditions.
The FCC requires that cable TV/LAN operators monitor their networks and calculate a Cumulative Leakage Index (CLI). The components of the CLI are the RF signal level, the distance from the coaxial cable, the frequency of the signal, and the number of leaks per coax mile. The CLI is calculated for a specific time period.
There are several licensed sources of RF energy in the frequency range that is used by the CATV industry for video. There are also unintended sources of RF energy in this frequency range such as power grids and commercial microwave ovens.
In the process of measuring the CLI it is essential to include signals which are true leaks from the CATV system. It is equally important to exclude those signals which are not leakage from the CATV system.
Previous efforts to solve this and related problems are covered in the following patents.
U.S. Pat. No. 5,294,937 (1994) to Osteen et al. discloses a cable leakage detection system to be placed in a moving vehicle. The system comprises a computer control and storage unit connected to a GPS Receiver, an RF meter and a hand held keypad. This device measures cable leakages and stores the data with time and location information on a floppy disk.
U.S. Pat. No. 4,413,229 (1983) to Grant discloses a method and apparatus for remote indication of faults in cable systems using an RF signal for fault detection.
U.S. Pat. No. 4,810,961 (1989) to Takahaski et al. discloses a system to determine the RF leakage from an electromagnetically shielded building. The system consists of transmitters with a different frequency for each floor of the building.
U.S. Pat. No. 4,072,899 (1978) to Shimp discloses an RF leak detector for detecting leakages from CATV coaxial cable lines. The RF leaks are detected by transmitting a unique signal and using a narrow band Receiver to detect the unique signal to thereby detect small amounts of RF leakages from defective CATV coaxial cables. If there is a leak, a unique recognizable audible tone will be detected.
U.S. Pat. No. 4,100,572 (1978) to Cochrane discloses a method and apparatus to test transmission lines using pulses of one nano-second duration.
A method used in the past to calculate the CLI is to use an RF detector and measure the field strength of signals in the frequency range of interest. However, because of the proliferation of RF generating equipment, such as two-way radios, florescent lights, and the like, erroneous detections of RF leaks are often made.
Another prior art method to detect leaks in the CATV system which eliminates the possibility of detecting erroneous signals is to transmit a unique signal on the CATV system. A narrow band Receiver is used to detect the unique signal. This method can be used to identify leaks from the CATV system in the presence of erroneous signals in the same frequency range. The disadvantage of this method is that it requires the transmission of the test signal over the CATV system. The spectrum available on the CATV system is increasingly limited, hindering the use of this method.
There is a need for a device that can positively identify an RF signal in the range of interest as a leak.
The present invention will detect and positively identify an RF signal as a leak from the CATV system. Further, the present invention will ignore RF signals that are not CATV leaks in the calculation of the CLI.
The present invention is an RF Receiver which can be mounted in a vehicle. The RF Receiver can be implemented with several options depending on the purpose of the vehicle (i.e. monitoring system health integrity or repair of defective components). The RF Receiver is a passive unit that requires no special test signals on the CATV system. The RF Receiver does not limit the speed of the vehicle.
An option of the invention used to aid in the location of the leak is an ability to determine if the leak is on the left or right side of the vehicle.